The present invention relates generally to packaged semiconductor devices and, more particularly, to lead frames used in the assembly of semiconductor devices.
FIG. 1 is a top view of one type of conventional quad-flat no-leads (QFN) lead frame 100. The lead frame 100 is a patterned sheet metal cut-out that includes a rectangular-shaped die paddle 102 (also known as a flag) surrounded by a ground bar 104, and metal leads 106 surrounding the ground bar 104.
FIG. 2 is a top view of a partially assembled, conventional QFN device 200. The QFN device 200 is assembled by adhesively mounting an integrated circuit (IC) die 202 on the die paddle 102 of the lead frame 100. The IC die 202 is then electrically connected to the leads 106 with bond wires 204 and the ground bar 104 with bond wires 206.
Following wire bonding, the sub-assembly including the IC die 202, the lead frame 100, and the bond wires 204 and 206 is encapsulated in molding compound (not shown). The encapsulation step includes putting the sub-assembly inside a mold form having a cavity, injecting uncured molding compound into the cavity, curing the molding compound, and then removing the mold form. Note, however, that the bottom surfaces of the leads 106 are left exposed so that the leads 106 may provide electrical connections between device-internal components on the die 202 and external components such as power sources and input/output connections on a printed circuit board (PCB) on which the QFN package 200 is mounted.
Although not depicted in FIG. 2, typically multiple QFN devices 200 are simultaneously assembled using a one- or two-dimensional array of the lead frames 100. After encapsulation, singulation is performed to separate the simultaneously assembled QFN devices 200 into individual packages ready for mounting on PCBs.
It is well known that during operation of a semiconductor device such as the QFN device 200, the components of the package expand and contract with changes in temperature. Due to differences between the coefficient of thermal expansion (CTE) of the molding compound and the lead frame 100, the molding compound may expand and contract at a different rate than the lead frame 100. These differences in expansion and contraction rates can cause stress on the adhesive bond between the molding compound and the lead frame 100.
To further complicate the issue, the adhesive bond between the molding compound and the lead frame 100 may be weakened due to, for example, contaminants trapped between the molding compound and the lead frame 100, or inherent weakness of (e.g., silver) plating on the surface of the lead frame 100. When these possibly weakened adhesive bonds are subject to expansion and contraction stresses, delamination can occur, where the molding compound separates from the lead frame 100.
Delamination between the molding compound and the lead frame 100 may result in one or more of the bond wires 204 and 206 lifting away from the lead frame 100 such that the electrical connection between the bond wire and the lead frame 100 is lost. Further, delamination can lead to cracking of the molding compound when (i) moisture becomes trapped between the molding compound and the lead frame 100 at the location of the delamination and (ii) the moisture expands due to temperature changes. Accordingly, it would be advantageous to have a semiconductor device that is less susceptible to delamination problems.